Generally, a GPS (global position system) receiver acquires GPS signals from a set of satellites and then tracks these GPS signals. During the acquisition stage, the initial carrier frequency of a carrier signal and initial phase of a pseudorandom noise (PRN) code associated with a received GPS signal are identified. These two parameters are then used for tracking the GPS signal.
Due to the motion of the satellites and the receiver, Doppler effects may occur and the carrier frequency and PRN code may vary over time. To overcome Doppler effects and maintain the availability of GPS signals, a tracking process is performed based on the initial carrier frequency and initial PRN code which are acquired during the acquisition stage.
When the variations of the carrier frequency and PRN code are successfully tracked by the receiver, the GPS signal can be referred to as “locked” by the receiver. When the receiver fails to track the variations of the carrier frequency and PRN code, the GPS signal from a corresponding satellite can be referred to as “lost”. When a signal is lost, it may not be used by the receiver for further processing such as calculating the position of the receiver. The receiver may need to perform acquisition again to ensure that there is enough number of GPS signals acquired and locked for further processing.
When the GPS signal is received by the receiver, bit boundaries of the navigation data stream can be identified after the carrier signal and PRN code have been stripped off from the received GPS signal. The navigation data stream is formed by a sequence of navigation data bits. The lasting time of each data bit can be 20 ms. The end of a data bit, which is also the beginning of another data bit, can be referred to as a boundary of a navigation data bit (also called bit boundary).
Conventionally, in a GPS receiver, a bit synchronization method is employed to determine if the GPS signal is locked or lost. More specifically, the bit synchronization module is needed to identify the bit boundaries of navigation data stream after the carrier signal and PRN code have been stripped off from the received GPS signal. According to such conventional bit synchronization method, the lock status of the GPS signal is detected at the same time when the bit boundaries of navigation data stream are determined. If the bit boundaries can not be determined after repeating the search process for a predetermined times, the signal is regarded as lost.
There are some drawbacks of this conventional method. First, the bit synchronization module may be difficult to determine the bit boundaries and the lock status if there are no data transitions within a relatively long bit sequence. In other words, this method may not be efficient when there are relatively long sequences of “0” or “1” in the navigation data stream. Second, this conventional method can be time-consuming. Third, the bit synchronization process may need to be performed from time to time in order to detect the lock status even after the GPS signal is acquired.
GPS signal power can indicate the quality of GPS signals. The carrier-to-noise power density ratio (CN0) in GPS receivers is defined as a ratio of the modulated carrier power to the noise power in a 1 Hz bandwidth. In decibels, carrier-to-noise power density ratio is expressed in dB-Hz. A greater carrier-to-noise power density ratio can indicate a higher signal quality. Carrier-to-noise power density ratio is a parameter in analyzing GPS receiver performance and may directly affect the precision of the receiver's pseudo range and carrier phase observations.
A traditional method to determine a carrier-to-noise power density ratio involves receiving a GPS signal, generating a noise signal and then calculating the signal power and the noise power to obtain the carrier-to-noise power density ratio. The traditional method may require extra hardware resources to calculate the noise power.